Field of the Invention
The present invention relates to hard disk drives (HDDs). More particularly, the present invention relates to a technique for setting the initial servo track pitch for a Self-Servo Writing (SSW) process.
Description of the Related Art
FIG. 1 shows an exemplary hard disk drive (HDD) 100 having a magnetic read/write head (or a recording slider) 101 that includes, for example, an offset head, that is positioned over a selected track on a magnetic disk 102 using a dual-stage servo system for writing data to and/or reading data from disk 102. Data is recorded in arrays of concentric data information tracks on the surface of disk 102. While HDD 100 shows only a single magnetic disk 102, HDDs typically have a plurality of stacked, commonly rotated rigid magnetic disks.
The dual-stage servo system of HDD 100 includes an actuator 105, a voice-coil motor (VCM) 104, for coarse positioning a read/write head suspension 106, and a secondary actuator, such as a microactuator or micropositioner, for fine positioning read/write head 101 over a selected track. As used herein, a microactuator (or a micropositioner) is a small actuator that is placed between a suspension and a slider and moves the slider relative to the suspension.
FIG. 2 depicts a cross-section of an exemplary suspension and microactuator arrangement 200 that includes a suspension 201, a microactuator 205 and a slider 209. Suspension 201 includes a load beam 202, a dimple 203 and a flexure 204. Microactuator 205 includes a substrate 206, a microactuator structure 207 and at least one flexure element 208. Substrate 206 is the stationary structure of microactuator 205. Microactuator structure 207 is the movable structure of microactuator 205. Slider 209 includes a read element 210 and a write element 211 that is offset from read element 201.
Track accessing and following is typically provided by a servo system of an HDD by using magnetically written patterns, referred to as servo patterns, that are stored on at least one magnetic disk of an HDD. One common type of servo pattern arrangement that is used is referred to as a dedicated servo system in which one surface of one of the hard disks is used for storing all the servo patterns. Another common type of servo pattern arrangement that is used is referred to as a sector servo system in which a small portion of a track between each sector or between several sectors on each track on each data surface is used for storing the servo patterns. Yet another common type of servo pattern arrangement that is used is referred to as a hybrid servo system in which both dedicated and servo-sector type servo system techniques are used, thereby obtaining advantages of each respective type of servo system.
One technique that is used for writing servo patterns on the disk or disks of an HDD uses special equipment referred to as a servo writer system. A servo writer system includes, for example, a laser-measured access system for accurately positioning the heads of the servo writer system over the disks of the HDD for accurately writing the servo patterns. The HDD is clamped to a servo writer in order to maintain accurate positioning between the HDD and the servo writer. U.S. Pat. No. 6,519,107 B1 to Ehrlich et al. discloses an exemplary a technique for writing servo patterns onto a magnetic hard disk drive.
One drawback of using a servo writer system is that it must be used in a clean environment in order to reduce the probability of contamination of the HDD because the HDD is open during the servo pattern writing process. Additionally, the resonances of the HDD change when the HDD is unclamped from the servo writer. Consequently, the servo system of the HDD does not perfectly follow the servo patterns, resulting in repeatable runout that makes determination of being on-track difficult by the servo system.
Self-servo writing (SSW) techniques have been developed for reducing the drawbacks associated writing servo patterns using a servo writer. For example, U.S. Pat. No. 6,040,955 to Brown et al. relates to a self-servo writing (SSW) technique in which servo information is written on a magnetic disk starting at a first crash stop of an HDD. The head writing the servo information is moved toward the other crash stop until the detected amplitude of the just-written servo information equals a predetermined amount, at which point more servo information is written. Movement of the head and writing of the next servo pattern continues across the disk until the second crash stop is encountered. U.S. Pat. No. 6,429,989 B1 to Shultz et al. relates to an SSW technique that writes timing marks across the surface of a magnetic disk based on detecting both the passage of the timing marks and writing radial extensions to timing marks at substantially the same circumferential positions.
One aspect of an SSW process is that the initial servo track pitch is set at the beginning of the SSW process. The compliance of the Inner Diameter (ID) crash stop and a predetermined amount of VCM current are used for producing a set of equally spaced tracks in a radial direction that are used as the basis for the radial propagation across the entire surface of the disk during the next phase of the SSW process. FIG. 3 shows a flowchart 300 for an exemplary conventional initial servo track-pitch-setting technique that is performed at the beginning of a conventional SSW process. At step 301, the motor driving the magnetic disks of the HDD is driven at the desired servowriter speed. At step 302, the actuator is unlatched from the ramp and the read/write heads are loaded onto the disk surface at a controlled speed. At step 303, the actuator is biased so that the read/write head is against the ID crash stop and the actuator is made ready for the SSW process. At step 304, burst patterns are written using a predetermined VCM current for a predetermined number of tracks, such as 16 tracks. Usually, 100-200 bursts are written per one disk revolution. For example, if the track has 200 sectors (sectors 0-199), a burst in written in each of sectors 0-199. The burst write timing and the VCM current are changed for each servo track.
FIG. 4 depicts the result of the exemplary servo initial track-pitch-setting technique shown in FIG. 3 after bursts are written for 16 tracks. FIG. 4 shows 16 servo tracks of two sectors, sectors 0 and 1. The lower portion of FIG. 4 is at the ID of the disk and the upper portion is toward the OD of the disk. Bursts b0-b15 have been written in each sector 0 and 1. Disk rotation is from right to left.
After the burst pattern has been written, the head is moved toward the innermost portion of the disk at step 305 and burst b0, i.e., the burst pattern located closest to the ID of the disk, is located using the read sensor of the read/write head. At step 306, the read/write head is moved toward outer diameter using very small steps of VCM current and bursts b1 and b2 are located. At step 307, the read sensor portion of the head is positioned over the center of burst b1 so that the amplitude of burst b0 equals the amplitude of burst b2 and the amplitude of burst b1 is a maximum. At step 308, the respective amplitudes of bursts b0, b1 and b2 are measured during several disk revolutions and averaged.
At step 309, the overlap is calculated, defined as Overlap=(b0+b2)/b1, in which b0, b1 and b2 are the respective averaged amplitudes of bursts b0, b1 and b2. At step 310, the head is moved toward the OD of the disk measuring and averaging the amplitudes of each burst b2-b14 and their respectively adjacent bursts, and calculating the overlap similar to the overlap calculation defined in step 309. For each measurement in step 310, the read sensor portion of the head is positioned over the center of the burst for which the overlap measurement is being made (i.e., the center of each burst b2-b14), so that the amplitudes of the bursts that are adjacent to the burst being measures are equal and the amplitude of the burst being measured is a maximum. At step 311, the calculated overlaps are compared to a target overlap value, such as 0.9. If, at step 311, the difference between the calculated overlaps and the target overlap value is within a selected criterion, such as 2%, the flow continues to step 312 and the initial track-pitch-setting technique is terminated.
If, at step 312, the calculated overlap is not within the selected criterion of the target overlap value, then flow continues to step 313 where it is determined whether the calculated overlap is greater than the target overlap value. If, at step 313, the calculated overlap is greater than the target overlap value, flow continues to step 314 where the predetermined VCM current interval is increased an increment. Flow continues to step 315. If, at step 313, the calculated overlap is less than the target overlap value, flow continues to step 316 where the predetermined VCM current overlap is decreased an increment. Flow continues to step 315 where all previously written bursts are erased. Flow continues to step 303 with the new predetermined VCM current and the process is repeated until the calculated overlap is within the selected criterion of the target overlap value.
At the end of initial servo track setting process of FIG. 3, a set of equally spaced tracks in the radial direction (i.e., 16 tracks having a few hundreds of burst patterns) have been created. The patterns are located at inner diameter portion of the disk. A conventional SSW uses the initial track-pitch-setting technique for compensating for a head having a large read/write offset. That is, the edge of the actuator touches the ID crash stop so no servo control is necessary for placing the head at the center of each burst. The number of written tracks must be greater than 2+Read/Write offset of the head in tracks because the read/write offset of the head is much greater than the servo track pitch, as depicted in FIG. 5. In FIG. 5, a head 501 includes read element 502 and write element 503, which are separated by read/write offset 504. Bursts 505 are shown written on servo tracks 506.
The conventional initial servo-track-pitch-setting technique relies on ID crash stop compliance for providing controllable, open-loop movement of the head. That is, when the actuator is pushed against the ID crash stop, i.e., step 303 in FIG. 3, the ID crash stop is compressed and the position of the head different than if the ID crash stop were not compressed. The position of the head when the ID crash stop is compressed is related to the amount of VCM current that is used for compressing the ID crash stop. FIG. 6 is a graph depicting the relationship between crash stop force (pushing force) in terms of VCM current as a function of actuator position. The VCM current values shown in FIG. 6 are representative and can change depending on the material used for the ID crash stop, the VCM torque constant, the geometry of the ID crash stop and the actuator, and other external forces. As FIG. 6 shows, ID crash stop compression, i.e., the position of the actuator, and the applied force are not linearly related. Typically, the compression range indicated by 601 is used during an initial servo track setting process. The curve of FIG. 6 is not exactly repeatable so head position at the ID crash stop is not exactly repeatable for the same VCM current. As a result, when a conventional initial servo track setting process is repeated, the process takes a lot of time. Further, additional time is required for the position of the head to settle as the head is moved across the disk based on VCM current. Moreover, the remainder of the SSW process is performed using the final predetermined fixed VCM current so the overall processing time and positional accuracy of the servo information can be adversely affected. Accordingly, expensive materials must be used for the ID crash stop because the ID crash stop compliance characteristics are critical for the initial track-pitch-setting technique.
Consequently, what is needed is a technique for setting the initial servo track pitch that does not rely on ID crash stop compliance characteristics as a basis for setting servo track pitch.